Lactation, a state common to all female mammals, is characterized by an inhibition of gonadotropin secretion. The primary hypothesis underlying the proposed experiments is that the neural systems activated by the suckling stimulus suppress GnRH activity, the neuronal system in the hypothalamus that governs female reproductive cyclicity. The goal of these studies is to identify neuronal systems that mediate the inhibitory effects of the suckling stimulus on GnRH neuronal activity. We have designed an acute resuckling model that emphasizes the neural effects of the suckling stimulus and reduces some of the complexity associated with lactation, such as chronic hyperprolactinemia and elevated levels of progesterone. In this model, the 8-pup litters are removed for 48 hrs. to allow for recovery from the effects of suckling. Reinitiation of suckling stimulus for 8-12 hrs. is effective in reimposing the inhibition on GnRH, indicating that neuronal systems inhibitory to GnRH are activated during this time. Because the suckling stimulus is applied acutely, immediate early gene expression, such as cFos, is induced in activated neurons, and therefore, can serve as a marker to identify activated populations of neurons. Using this technique, we have identified the following areas in the hypothalamus that are specifically activated by acute resuckling: the periventricular preoptic area, medial preoptic area, anterior hypothalamus and paraventricular nucleus. We propose that putative inhibitory neuronal subtypes in these areas are the most likely candidates for mediating the inhibition of GnRH activity: Similarly, we propose that the catecholaminergic systems in the brainstem, which are also activated by acute resuckling, are the most likely candidates for relaying the effects of suckling to the hypothalamus. SPECIFIC AIM 1 will identify specific neuronal pathways activated in response to the suckling stimulus that mediate the suppression of GnRH neuronal activity. Experiments include determining the projections of the activated neurons in the periventricular preoptic area, medial preoptic area, anterior hypothalamus and paraventricular nucleus to areas of GnRH cell bodies and identifying the phenotype of activated neurons of interest. Putative inhibitory neuronal subtypes, such as GABA or CRF, will be identified by determining the effectiveness of specific antagonists in reversing the inhibitory effects of the suckling stimulus on GnRH. These studies will provide for the first time detailed information about the specific afferent neuronal pathways mediating GnRH neuronal activity and will identify the neuronal systems in the brainstem that relay the signals from suckling to the hypothalamus. SPECIFIC AIM 2 will assess whether the increase in NPY neuronal function within the arcuate nucleus during lactation contributes to the inhibition of GnRH neuronal function. The proposed inhibitory effects of NPY will be assessed by using pharmacological probes to inhibit its activity. Tract-tracing techniques will be used to determine the projections of the activated NPY neurons, which will provide information as to their site of action. An understanding of the mechanisms by which the suckling stimulus imposes an inhibition on GnRH neuronal function will provide information that is relevant to other hypothalamic causes of infertility and may suggest new approaches for contraception.